(a) Technical Field
The present invention relates to a method for opening an engine clutch of a hybrid vehicle. More particularly, the present invention relates to a method for more easily opening an engine clutch of a hybrid vehicle without a physical slip using a hybrid starter generator (HSG) torque converged to an engine friction torque.
(b) Background Art
A hybrid vehicle among environmentally-friendly vehicles is a vehicle which uses an engine and a motor as a power source to reduce exhaust gas and enhance fuel efficiency and includes a power transfer system which individually transfers power from the engine or power from the motor to a driving wheel or transfers power from both the engine and the motor to the driving wheel.
As illustrated in FIG. 1, a power transfer system for a hybrid vehicle according to the related art includes, for example, an engine 10 and a motor 12 which are disposed in series, an engine clutch 13 disposed between the engine 10 and the motor 12 to transfer or interrupt power of the engine, an automatic transmission 14 configured to shift power from the motor or power from both the motor and the engine to a driving wheel and output the power, a hybrid starter generator (HSG) 16 which is a type of motor connected to a crank pulley of the engine to start the engine and generate power, an inverter 18 configured to operate the motor and controls power generation, a high voltage battery 20 chargeably and dischargeably connected to the inverter to provide power to the motor 12, and the like.
The power transfer system for a hybrid vehicle is a type of system in which the motor is attached to the automatic transmission and is referred to as a transmission mounted electric device (TMED) and provides various driving modes, such as an electric vehicle (EV) mode which is a pure electric vehicle mode using only the power from the motor, a hybrid electric vehicle (HEV) mode using the engine as a main power source and the motor as an auxiliary power source, and a regenerative braking (RB) mode which recovers braking and inertial energy of a vehicle based on the power generation by the motor when the vehicle is driven by braking or inertia of the vehicle and charges the recovered braking and inertial energy in the battery.
In the HEV mode, the vehicle is driven by a sum of output torques of the engine and the motor when the engine clutch is locked-up. In the EV mode, the vehicle is driven by the output torque of the motor when the engine clutch is locked-up and opened. In particular, to prevent a physical slip from occurring, the engine clutch is required to be locked-up when an engine speed is synchronized with a motor speed during a driving mode shift from the EV mode to the HEV mode, while the engine clutch is required to be opened when an input shaft torque of the engine clutch is in a zero state during the driving mode shift from the HEV mode to the EV mode.
The engine speed is synchronized with the motor speed when the engine clutch is locked-up and the input shaft torque (hereinafter, input torque) of the engine clutch connected to the engine is adjusted to zero when the engine clutch is opened to prevent a physical slip of the input shaft of the engine clutch with respect to the output shaft of the engine clutch connected to the motor side.
In particular, the reason for adjusting the input torque of the engine clutch be zero (0) to prevent the physical slip from occurring when the engine clutch is opened is as follows. First, it may be difficult to compensate for a slip rate of the engine clutch in the motor side by accurately estimating a change in a transfer torque of the engine clutch during the slip of the engine clutch and second, since the slip of the engine clutch occurs when an absolute value of the input torque of the engine clutch is greater than torque capacity by pressure (e.g., oil pressure) applied to the engine clutch, the input torque of the engine clutch is required to be zero when the pressure of the engine clutch is opened.
A method for opening an engine clutch according to the related art will be described below with reference to FIG. 2. To prevent the physical slip of the engine clutch, the related art maintains fuel injection into the engine from the time when an opening of the engine clutch starts to adjust until the engine torque, that is, an input torque of the engine clutch becomes zero (0) and opens the locked-up pressure (oil pressure) of the engine clutch (E/C) when the input torque of the engine clutch becomes zero to open the engine clutch.
In particular, the input torque of the engine clutch becomes zero after a predetermined time lapses due to a friction torque (e.g., torque generated by a friction force generated from each sliding portion of the engine) of the engine when the engine is operated in a low torque area due to the fuel injection into the engine. When the engine clutch is opened, the fuel injection into the engine stops.
However, the method for opening an engine clutch according to the related art has the following problems. Usually, it is known that operation efficiency of the engine approaches an optimal operating line (OOL) and thus the operation efficiency of the engine is sufficient, while it is known that operation efficiency of the low torque area of the engine is distant from the optimal operating line and thus the operating efficiency thereof is substantially reduced.
Therefore, the operation of the engine is maintained in the low torque area by maintaining the fuel injection into the engine from the time when the opening of the engine clutch starts to adjust until the engine clutch is opened, during the shift from the HEV mode to the EV mode, which may be a cause of the reduction in fuel efficiency depending on fuel consumption.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.